1. Field of the Invention
The present invention relates to a ferrite stainless steel superior in thermal fatigue strength usable for a muffler, exhaust manifold, or other automobile exhaust system member.
2. Description of the Related Art
Due to the rise in interest in environmental issues, improvement of the fuel economy of automobiles and in turn reduction of the weight of vehicles are being strongly demanded. Further, purification of the exhaust gas of vehicles is earnestly desired. In view of this, stainless steel has been used for automobile exhaust system members. One of the members exposed to the highest temperatures, that is, the exhaust manifold, is repeatedly subjected to a rise in temperature to as high as 1000° C. and a drop in temperature to ordinary temperature, so requires superior heat resistance, in particular thermal fatigue strength.
In recent years, the usage temperature of exhaust manifolds has risen further. Steels are therefore being developed which can handle temperatures of 950° C. For example, Japanese Unexamined Patent Publication (Kokai) No. 06-100990 discloses a stainless steel containing 18 to 22% of Cr, 1.0 to 2.0% of Mo, and 0.1 to 1.0% of Nb. At the present time, as a 950° C. compatible exhaust manifold material, 19% Cr-2% Mo-based and other ferrite stainless steels called “SUS444” steels in JIS standard are being used.
The superior high temperature strength of ferrite stainless steel is considered to be due to the strengthening of the dissolved Nb and Mo contained in the steel. If exposed to a high temperature over a long period of time, however, the dissolved Nb and Mo precipitate, so the dissolved amounts fall and the high temperature strength ends up falling, that is, the thermal fatigue strength falls. As a method for preventing such a drop in high temperature strength, Japanese Patent No. 3021656 discloses to add Nb and Ti together to suppress the precipitation of the Nb.
Further, Hirasawa et al., CAMP-ISIJ, vol. 16 (2003), p. 544 reports that when reducing the amount of Si in a 14% Cr—Mo—Nb ferrite stainless steel from 0.9% to 0.35%, the dissolved Mo increases and the high temperature strength rises. Further, Japanese Patent No. 3242007 discloses a ferrite stainless steel for an automobile exhaust system member superior in oxide scale resistance. This steel improves the oxide scale resistance by lowering the Si.
However, it has been found that the problem of the drop in the high temperature strength after high temperature aging and the drop in the thermal fatigue strength cannot be sufficiently solved even with the technology described in Japanese Patent No. 3021656. Further, the high temperature strength dealt with in Hirasawa et al. CAMP-ISIJ, vol. 16 (2003), p. 544 is the initial high temperature strength. That publication does not disclose anything means for solving the problem of the thermal fatigue strength. Further, Japanese Patent No. 3242007 does not describe anything regarding thermal fatigue strength. In addition, the steel of that patent is low in Si and free of Al and therefore has almost no deoxidizing elements, so has the problem of extremely difficult deoxidation and precise control of composition.